…y version

* 'returns null for empty' now actually checks empty string
  (previously this branch was ignored in test coverage)

* 'returns null for non-object YAML' - existing test renamed
  (previously named 'for empty' which isn't what it was checking)

There is a test ('returns null for non-object YAML') for this behaviour,
but it was relying on the loop iterating over individual characters in
the resulting string and not finding anything useful in them,
which seems more like an accident than the way things were intentionally
structured. So, this adds an explicit check for this case.

This was previously necessary to get the test 'returns null for
non-object YAML' to pass, but that seems to have been accidental and was
fixed in the previous commit.

This does change the behaviour of this function, but not (I think) in a way
which will cause issues: it will now return an empty array rather than null
in the case of a compose file that has no services with an image, but doing
so makes sense: the file was extracted successfully, it's just that there's
nothing of interest to be found in it.

Closes renovatebot#6570.